In conventional systems for optical earth observation in a single spectral channel, only a single telescope with one detector array in its focal plane is used. In order to improve performance, i.e., ground resolution, it was necessary to use telescopes with focal lengths as large as possible and simultaneously with an aperture that was as large as possible, assuming constant altitude and size of the detector element.
As is known, focal length is linearly related to height above the ground as well as to resolution. Hence, a doubling of resolution requires a doubling of the focal length and therefore a doubling of the structural length for the same telescope construction. Aperture size is also linearly related to resolution by diffraction. Therefore, for a higher resolution, the size of the aperture of the telescope objective lens must also be increased.
Further, the size of the objective lens, which serves as the light-collecting surface of the optical system, is dependent on the size of the reflecting terrain surface segment or area for a given signal-to-noise ratio. Thus, if the area of the terrain surface segment being visualized is reduced by half, then, for a constant signal-to-noise ratio as well as for a constant exposure time, the area of the aperture must be doubled. Also, the available exposure time, for example, in the case of a satellite in a low orbit, is proportional to the linear dimension of the terrain surface segment in the flight direction, due to the velocity of the satellite of approximately 7 km per second relative to the ground. If the length of the terrain surface segment in the flight direction is reduced by one-half, the exposure time is also reduced by one-half.
The aperture size thus increases with increasing resolution of the telescope, as does focal length. This increases the volume and mass of the telescope and correspondingly increases costs.